The present invention relates to a new and improved apparatus for controlling the quality of a picture which is to be processed in a reproduction process, and also pertains to a method of producing such apparatus.
Original pictures normally show continuous tones. For example, paintings, photographs and slides are continuous tone originals. However, normal printing procedures are not able to reproduce continuous tones and screened prints are instead produced.
In order to arrive at a screened print from a continuous tone original, five principal steps are performed.
The first step is the production using color separation techniques of a continuous tone reproduction for each of the primary colors namely magenta, cyan and yellow. A fourth continuous tone reproduction is also normally produced, for the color black, in order to improve contrast. The continuous tone reproductions, which are continuous tone black-and-white transparencies, are produced using a reproduction camera or a contact printing method.
The second step is the screening of the three or four continuous tone reproductions. This screening is done using a reproduction camera in contact with a screening plate. The effect of this step is to produce, from the continuous tone reproductions, original screened films in which the continuous tones appear as half-tone dots. Generally, the screen dots produced in this second step are not sharp enough, and the third step is carried out in order to sharpen the dots.
In the third step, film contact of the original screened film is carried out to produce contact films.
In the fourth step, a printing plate is made from the contact films. In offset printing, the printing plate is normally metallic, and the metallic plate is covered with an emulsion which can be sensitized. The half-tone image carried by the contact films is transferred to the printing plate by a contact printing method using a very hard metal-halogen light. After exposure, the emulsion on the metallic plate is developed and finished, and the screen dots of the contact films are transferred to produce screen dots on the metallic plate.
In the final step, the printing plate is used in the printing press to produce screened prints.
The quality of screened prints obtained in a reproduction process such as that described above is influenced by different factors, such as resolution power, tone displacements, screen dot deformation, color layer thickness, grey balance. In order to be able to more readily control the complex operations which arise during performance of such process attempts have been made to render the individual influencing factors independent of the picture which is to be produced and to determine such separately.
It is known for this purpose to use control devices consisting of measuring of measurement symbols assembled together into a group. These control devices are transported externally of the actual pictures, however upon the same picture carrier, past one or a number of work stations of the reproduction process. In this way it is possible to separately determine the individual influencing factors independent of the picture subject, because there is known ahead of time the picture quality which is to be obtained of the measurement symbols of such control devices. From the changes in the measurement symbols of the control device which have been ascertained it is possible to draw conclusions concerning corresponding changes of the screen dots at the printing form and at the printed picture. Not only is there simplified the checking of the course of the reproduction- and printing process, but the control thereof is also thereby simplified and the quality of the picture which is to be produced is rendered more uniform.
In order to be able to obtain the data necessary for determining the different influencing factors in sufficient amounts, the control devices must possess a suitable construction and be distributed in sufficient numbers over the printing form. For practical reasons, the control devices are combined in a selected arrangement into so-called measurement strips, also known as measurement beams. Since the transport of the relevant control device i.e., the entire measurement beam as a general rule requires an increase in the necessary paper format and thus an additional expenditure in material, attempts have been made for reasons of economy to maintain, on the one hand, the control device, as small as possible and, on the other hand, to construct such that they can deliver as much data as possible.
Control devices are used for instance for visual, microscope and densitometric controls. Visually, if necessary with the aid of a magnifying glass and a microscope, there are evaluated on the basis of appropriate control devices the color superimposition, the grey balance, slurring and doubling, the increase in tone grade and the peak point tone grade. Densitometrically there are measured color density and tone displacements as a consequence of the deformation in the form of dots.
The control devices which have heretofore been employed and the thus employed measuring techniques possess a series of shortcomings and disadvantages:
1. Each of the control devices in a measurement beam was previously constructed for evaluating only a single characteristic. Therefore there were required a large number of different control devices in a measuring or measurement beam and such thus requires a greater amount of space. PA0 2. For the densitometric measurement the control devices must possess a minimum size, determined by the measurement spot of the employed device. For each tone value stage there is thus required an additional control device with a larger number of equal size measurement symbols. PA0 3. The measurements with a measurement microscope, although possessing the advantage of being able to get by with control devices of very small dimensions, in fact in some instances with individual measurement symbols, nonetheless possesses the drawback that for practical requirements it is much too inaccurate for screen stages of 20% to 80% surface coverage. The screen dots which exist during the standard screening of pictures generally possess shapes, the surface coverage of which is difficult to calculate.